Orthogonal frequency division modulation (OFDM) is a promising technique in optical fiber communication for its high-spectral efficiency, the ability to combat both chromatic dispersion (CD) and polarization mode dispersion (PMD), and the flexibility in digital signal processing. (Please refer to Jean Armstrong, “OFDM for Optical Communications”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 3, Feb. 1, 2009; Dayou Qian, Neda Cvijetic, Junqiang Hu, Ting Wang “Optical OFDM Transmission in Metro/Access Networks”, OSA/OFC/NFOEC 2009, OMV1; A. J. Lowery and J. Armstrong, “Orthogonal-frequency-division multiplexing for dispersion compensation of long-haul optical systems,” Opt. Express, vol. 14, no. 6, pp. 2079-2084, 2006; Sander L. Jansen, Itsuro Morita, Kamyar Forozesh, Sebastian Randel, Dirk van den Borneland Hideaki Tanaka, “Optical OFDM, a hype or is it for real?” ECOC 2008, Mo.3.E.3; D. Qian, N. Cvijetic, Junqiang Hu and Ting Wang, “108 Gb/s OFDMA-PON with Polarization Multiplexing and Direct Detection”)
In recent years, lots of researches focused on the coherent detection OFDM (CD-OFDM) since it has the advantages of high spectral efficiency, and high receiver sensitivity. However, due to its complex structure and high-bandwidth hardware requirement, it will be very expensive. (Please refer to Hidenori Takahashi, “Coherent OFDM Transmission with High Spectral Efficiency,” ECOC 2009, paper 1.3.3; W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design)
Compared with optical CD-OFDM, optical direct detection (DD) OFDM is a more affordable scheme for its simple receiver. But in order to avoid interferences between the optical carrier and OFDM signals, a spectral-inefficiency frequency guard band (FGB) is needed between the optical carrier and OFDM subcarriers in the traditional optical DD-OFDM, and many investigations have been done for improving the spectral efficiency of the optical DD-OFDM. (Please refer to Wei-Ren Pengl, Xiaoxia Wu, Vahid R. Arbab, Bishara Shamee, Jeng-Yuan Yang, Louis C. Christen1, Kai-Ming Feng, Alan E. Willner and Sien Chi, “Experimental Demonstration of 340 km SSMF Transmission Using a Virtual Single Sideband OFDM Signal that Employs Carrier Suppressed and Iterative Detection Techniques”, OFC/NFOEC 2008, OMU1; Abdulamir Ali, Jochen Leibrich and Werner Rosenkranz, “Spectral Efficiency and Receiver Sensitivity in Direct Detection Optical-OFDM”, OSA/OFC/NFOEC 2009, OMT7; Arthur James Lowery, “Improving Sensitivity and Spectral Efficiency in Direct-Detection Optical OFDM Systems” OFC/NFOEC 2008, OMM4; Zhenbo Xu, Maurice O'Sullivan and Rongqing Hui, “Spectral-efficient OOFDM system using compatible SSB modulation with a simple dual-electrode MZM” OSA/OFC/NFOEC 2010, OMR2; Zizheng Cao, Jianjun Yu, Wenpei Wang, Lin Chen, and Ze Dong, “Direct-Detection Optical OFDM Transmission System Without Frequency Guard Band”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 11, Jun. 1, 2010)
Additionally, Turbo coding can be used to improve the receiver sensitivity, and is used widely in wireless system. (Please refer to Lajos Hanzo, Jason P. Woodard, and Patrick Robertson, “Turbo Decoding and Detection for Wireless Applications”)
Therefore, it is desired to improve the receiver sensitivity of the optical DD-OFDM system while obtaining high spectral efficiency with simple architecture.